A polyester film, particularly a biaxially oriented polyethylene terephthalate film and a biaxially oriented polyethylene naphthalate film are widely used, due to their excellent mechanical properties, heat resistance and chemical resistance, as raw materials for magnetic tapes, ferromagnetic thin film tapes, photographic films, packaging films, films for electronic parts, electrical insulating films, films for laminating metal plates, films to be stuck on surfaces of glass displays, and protective films for a variety of members.
In recent years, a polyester film is often used for a variety of optical applications. For example, it is used as base films for a prism lens sheet, touch screen and backlight which are members for a liquid crystal display device, a base film for an antireflective film and a base film for an explosion-proof film (used to prevent glass from scattering upon implosion of CRT) for a display. Base films used for such optical applications must have excellent transparency and excellent easy adhesions to a prism lens, a hard coat layer, an adhesive layer and an antireflection layer.
The polyester film generally has poor adhesion to a prism lens or hard coat layer comprising a material other than a polyester, e.g., an acrylic resin, as a main component. To improve the poor adhesion, it is proposed to laminate an easily adhesive layer using a polyester resin, an acrylic resin or an urethane resin on surfaces of the polyester film (refer to JP-A 10-119215 and JP-A 2000-246855). However, easily adhesive layers proposed in these publications may exhibit insufficient adhesions depending on applications. For example, the easily adhesive layers have such a problem that when a polyester film having the easily adhesive layers formed thereon is used as a film for CRT, the film exhibits insufficient adhesion to an adhesive layer on the other side of the film while exhibiting good adhesion to a hard coat layer.
Further, a filler (lubricant) is generally introduced into the polyester film so as to improve slipperiness (easy slipperiness) of the film. However, when the polyester film is used as a base material for optical applications, an amount of filler to be added must be minimized since transparency is required. However, when the amount of the filler to be added is small, the surfaces of the film become flat, so that there arises a problem that the surfaces of the film stick to each other when the film is rolled up so as to make the surfaces of the film contact with each other and the stuck portions may become surface defects of the film when the film is unrolled. Further, since slipperiness between the surfaces of the film is poor, the film is difficult to handle, so that there occurs a problem that scratches are formed on the surfaces of the film during production or processing of the film. These problems are particularly noticeable when a resin having a low glass transition point is used in the easily adhesive layers so as to improve adhesion of the polyester film.
Meanwhile, in recent years, use of the polyester film as a surface protecting material for window glass, a showcase, glasses, a meter, a display and a lamp has been receiving attention. To be used for such a purpose, the polyester film must have excellent surface hardness and abrasion resistance as well as sufficient transparency and antireflectivity.
To satisfy such a requirement, it has been attempted to laminate a hard coat (HC) layer and an antireflection (AR) layer on the polyester film. However, due to lack of adhesion to the polyester film, a satisfactory result is not obtained, yet.
As a method for improving the adhesion of the polyester film, for example, a method in which an aqueous polyester having a glass transition point of 40 to 85° C. is coated on a base film of a transfer film for in-molding so as to improve adhesion to a medium layer is known (refer to JP-A 7-156358).
However, although the adhesion between the base film in the transfer film for in-molding and the medium layer is improved by the method, adhesion in other applications is often not improved to a sufficiently satisfactory level.
Meanwhile, to obtain an easily slippery polyester film having flat surfaces, it is known to form a coating film comprising a composition containing a polyurethane or acrylic resin and a fatty acid amide or bisamide on surfaces of a polyester film (refer JP-A 63-194948).
However, the publication does not indicate that adhesion is improved by use of the fatty acid amide or bisamide.
Further, it is also known that a laminate having good surface hardness and abrasion resistance is obtained by forming a polyester resin layer on a polyethylene terephthalate layer and then forming a radiation curable layer having specific composition on the polyester resin layer (refer to JP-B 7-80281). However, the laminate lacks general versatility due to use of the special curable layer and is not fully satisfactory in terms of adhesion.
In recent years, due to rapid proliferation of personal computers, demand for an antiglare (antireflection) transparent board for a personal computer display which provides good visibility over a long time, displays images with high hue contrasts, causes little exhaustion on an operator and is not easily scratched when wiped to be cleaned has been increasing, and improvements of the above technologies are desired.